One of the problems encountered in optical switching is the buffering of signals. To switch time-division-multiplexed signals, it is almost always necessary to interchange the time slots and, hence, to buffer the signals. In packet switches, which are currently of particular interest with regard to future ATM systems (ATM = asynchronous transfer mode), signals have to be buffered whenever two packets, called "cells" in ATM, are present simultaneously for transfer in a particular channel. Because of the random traffic volume, the possibility of two packets having to be forwarded simultaneously on the same path cannot be ruled out.
From J.S. Turner, "Design of a Broadcast Packet Network", published in "Proceedings of INFOCOM '86", April 1986, pages 667 to 675, it is known to combine Links of an ATM switching network into link groups. The links of each group run parallel and are completely equivalent. Besides increasing capacity, this compensates for random traffic variations, which reduces the number of buffers required.
DE 37 42 941 C2 (corresponding to U.S. Pat. No. 4,922,487 discloses an ATM exchange in which fixed numbers of cells or packets to be switched are combined into a frame, and all packets are divided into subpackets of equal length and distributed to subframes. Switching takes place on the basis of subframes using synchronous time-division multiplexing. Use is made of the fact that all time slots on a line represent equivalent serving channels, whereby random variations are largely compensated for. The buffers are those which are required for synchronous time-multiplexed switching anyhow. The storage space required in the switching network is reduced in the ratio of frames : subframes.
From D. W. Smith et al, "Multidimensional Optical Switching Networks", IEEE 1989it is known to combine the conventional multiplexing methods, which, in optical switching, also includes wavelength- or frequency-division multiplexing besides time- and space-division multiplexing, for the purpose of increasing the capacity of an optical exchange. The addition of wavelength- or frequency-division multiplex as a further dimension increases not only the capacity of the exchange but also, to the same extent, the number of equivalent serving channels. However, the total capacity of this exchange is quite limited.
With a sufficient number of equivalent serving channels, random variations are compensated for to the point that buffers can be dispensed with except those required for time-slot interchanging during time-division multiplexing. Even in those cases where buffers are used, there are Limits to the compensation of random variations.